In the automobile industry, "progressive springs" for small automobiles, are coiled from tapered bars, so that the "ride" for four people in a small car is essentially the same as for one person. Some bars may be tapered in one direction for the entire length, and other bars may have symmetrical, or reverse tapers at opposing ends. One example of such springs is depicted in U.S. Pat. No. 4,077,619. The bars may be produced by several conventional bar forming processes, but to produce the size and finish required for performance and fatigue life, grinding is the preferred process for manufacturing. Ideally, these bars should be taper-turned then ground for the final operation before going into the coiling furnace, because turning inevitably introduces a serious compromise between efficient utilization of power and good surface finish, whenever finish is an important parameter of the operation. Turning, however, may become a problem when it becomes necessary to remove large amounts of stock from relatively small diameter bars, as will be the case when turning the small diameter of a tapered bar. This comes from the fact that the torque must be absorbed in "wind-up" of the bar, which limits the amount of horsepower that can be applied. It therefore becomes of paramount interest to consider grinding spring bars from the solid on a production basis. The centerless grinder has enjoyed great success in its duty as a high volume production machine, but prior art centerless grinder configurations are generally unsuitable to perform the task of generating spring bar cross-sections on a production-type through-feed grind set-up.
For the last 25 or 30 years, tapered fishing rods have been ground on a centerless grinder. On these rods, the taper is generated by synchronizing the infeed movement of the standard slide with the thrufeed rate produced by the regulating wheel rotation and feed angle. This basic concept could just as well be applied to grinding tapered steel bars, except for a fundamental difference in the behavior of a grinding wheel when it is grinding plastic impregnated fiberglass material and when it is grinding steel. When grinding plastic rods, the wheel does not wear significantly, even when grinding an extreme taper out of a cylindrical blank. In most steel bar grinding, in order to minimize the machine cost of the operation, we must use wide wheels and high stock removals. The high stock removals result in relatively high rates of wheel wear (G-ratios, i.e. the ratio of stock removal to wheel wear, of "6" are considered quite good and are frequently much less). Consequently, "self-dressing" wheels are used, because the time of the dressing operation on wide wheels, the frequency due to the high wheel wear rates, and the cost of diamonds, make diamond dressing prohibitively expensive.
A self-dressing grinding wheel is a wheel which, because of its construction, during grinding exhibits uniform, continuous, controlled breakdown of the grinding surface to present sharp cutting elements continuously during the grinding operation.
On fishing rod grinders, where the wheel axes remain parallel during the slide infeed, both wheels are profiled, with one part of the profile removing the material and the other parts generating the surfaces on the tapered and on the straight portions of the rod. If this straight infeed method were used for grinding steel, the wear action could so quickly deteriorate the wheel profile that in a matter of minutes redressing would be required, and the operation could not possibly prove viable.
Applicant has obviated the difficulties inherent in the prior art design by a novel grinding apparatus and method.
It is therefore an object of the present invention to provide a machine structure capable of producing tapered spring bars on a through feed centerless grinding setup.
Another object of the present invention is to provide a centerless grinding machine structure capable of generating varied workpiece cross-section diameters by relative pivotal movement of regulating and grinding wheels in timed relationship to through-feed movement of a workpiece.